Carbon fiber/silicon carbide (C/SiC) composites have the potential to overcome the shortcomings of the currently used carbon and metallic friction materials in aircraft brakes. In the present work, chemical vapor infiltration (CVI), melt infiltration (MI) and polymer impregnation and pyrolysis (PIP) were used to fabricate materials for mechanical, thermal and frictional tests. PIP materials exhibited the best mechanical properties, followed by MI and CVI materials. In the case of thermal testing, MI materials were superior to the other materials. C/SiC friction coefficients were higher than the values obtained for a typical carbon composite. Wear rates of the ceramic materials were comparable to the values obtained for C/C.
An internally consistent set of data was generated for the chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS) and H2 at atmospheric pressure. A moving fiber tow was used as the substrate. Coating rates between 0.3 and 3.7 µm/min and deposition efficiencies between 24 and 48% were obtained for MTS and H2 flow rates in the range 30 to 200 cm3/min and 300 to 2000 cm3/min, respectively. The data were analyzed and found to be best fit under a mass transfer regime. Based on this fit, a value of the constant in the Chilton–Colburn j factor expression for a moving fiber tow was estimated to be 2.74 × 10−6 with a standard deviation of 3.2 × 10−7. The efficiency of the reaction was found to decrease with increases in the total flow rate, indicating that the effect of the decreased residence time of reagents in the reactor was larger than the increase in the mass transfer coefficient. Finally, a comparison between the efficiencies for a stationary and a moving tow revealed that the moving tow had a higher efficiency, possibly due to a disruption of the boundary layer by the tow motion or due to the decrease in the canning of the moving tow.
Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. The preforms for the infiltration were prepared by stacking 40 layers of carbon cloth in a graphite holder. The preforms were infiltrated with carbon using propylene or methane as a reactant, with hydrogen as a diluent. Composites with porosities as low as 7% have been processed within 8-12 h. The highest deposition rate obtained in the present study was ∼3 μm/h, which is more than an order of magnitude faster than the typical value of 0.1-0.25 μm/h for the isothermal infiltration process.
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